The cohesion tension theory is introduced by Dixon and Joly in 1894, which is also known as “Suction due to transpiration”. Basically this theory explains the ascent of sap.
As per this theory there are two essential characteristics of the plant:
As per the cohesion theory, the similar molecules of water possess the attraction force that is very high more than 1000 atmosphere, this attraction force is known as cohesion force that helps in the transportation of water molecules towards the tip of the plant.
There is also an attraction between the water molecule and the inner wall of xylem.
The upward transportation of water molecules is basically due to the process of transcription where one molecule of water is evaporated and other molecule of water is pulled towards the arial part of the plant by the process of cohesion.
By this whole process of water lifting, the negative tension is occurred from the aerial parts to the base of the plant.
Due to this there is a continuous movement of water molecules in the opposite direction of the negative tension, which is known as the transpiration pull.
This process of cohesion and transpiration pull supports the movement of water channel towards the aerial part of the plant by opposing the gravitational force.